1. Field of the Invention
The present invention relates to an electromagnetic relay for use in activating and controlling a direct current (DC) motor for driving a windshield wiper drive section or a power window drive section of automobiles, for example.
2. Description of the Prior Art
Heretofore, DC motor drive circuits using an electromagnetic relay have often been used in order to activate and control a windshield wiper drive section and a power window drive section of automobiles.
FIG. 1 of the accompanying drawings is a schematic circuit diagram showing an example of a prior-art DC motor drive circuit for use in a windshield wiper drive section. FIG. 2 is a schematic circuit diagram showing an example of a prior-art DC motor drive circuit for use in a power window drive section.
First, an example of the DC motor drive circuit for use in the windshield wiper drive section will be described with reference to. FIG. 1.
As shown in FIG. 1, one end of a windshield wiper driving DC motor 1 is connected to a terminal 2a connected to a movable contact (this movable contact is usually connected to a suitable means such as a contact spring driven by an armature) AR of an electromagnetic relay 2. The above terminal 2a connected to the movable contact AR will hereinafter be referred to as a xe2x80x9cmovable contact terminalxe2x80x9d.
The other end of the DC motor 1 is connected to a terminal 2b connected to a normally closed contact N/C (i.e. break contact) of the electromagnetic relay 2. The above terminal 2b connected to the normally closed contact N/C will hereinafter be referred to as a xe2x80x9cnormally closed contact terminalxe2x80x9d. A connection point 2d between the other end of the DC motor 1 and the normally closed contact 2b is connected to the ground.
A terminal 2m connected to a normally open contact N/O (i.e. make contact) of the electromagnetic relay 2 is connected to a power supply at a terminal 3, at which a positive DC voltage (+B) is supplied from a car battery. The above terminal 2m to which the normally open contact N/O is connected will hereinafter be referred to as a xe2x80x9cnormally open contact terminalxe2x80x9d.
The electromagnetic relay 2 includes a coil 2C. The coil 2C is energized or de-energized by control current supplied from a windshield wiper drive controller 4 when a user operates a windshield wiper switch 5. This windshield wiper switch 5 includes three fixed contacts 5a, 5b, 5c and a movable contact 5m. 
When the windshield wiper switch 5 connects the movable contact 5m to the fixed contact 5a (xe2x80x9cOFFxe2x80x9d position), the coil 2C is not energized by controlling current from the windshield wiper drive controller 4 so that the electromagnetic relay 2 connects the movable contact AR to the normally closed contact N/C. As a result, one end and the other end of the DC motor 1 are connected to each other and thereby the DC motor 1 can be braked (or placed in the stationary state).
When the windshield wiper switch 5 connects the movable contact 5m to the fixed contact 5b (xe2x80x9cINTERMITTENTxe2x80x9d position), the coil 2C of the electromagnetic relay 2 is intermittently energized by the controlling current from the windshield wiper drive controller 4. As a result, the electromagnetic relay 2 connects the movable contact AR to the normally open contact N/O while the coil 2C is being energized by the control current. When the coil 2C is not energized by the control current, the electromagnetic relay 2 connects the movable contact AR to the normally closed contact N/C. Specifically, the electromagnetic relay 2 alternately connects the movable contact AR to the normally closed contact N/C and the normally open contact N/O each time the coil 2C is energized or is not energized.
When the electromagnetic relay 2 connects the movable contact AR to the normally open contact N/O, direct current flows through the DC motor 1 as shown by a solid-line arrow I in FIG. 1 and thereby the DC motor 1 can be driven. When the electromagnetic relay 2 connects the movable contact AR to the normally closed contact N/C, the supply of the direct current I to the DC motor 1 is interrupted and the DC motor 1 becomes a generator of direct current so that direct current flows through the DC motor 1 in the direction opposite to that of the direct current I and the DC motor 1 can be braked, i.e. the DC motor 1 can be driven intermittently. As this DC motor 1 is driven intermittently, the windshield wiper is driven intermittently.
When the windshield wiper switch 5 connects the movable contact 5m to the fixed contact 5c (xe2x80x9cCONTINUOUSxe2x80x9d position), the coil 2C of the electromagnetic relay 2 is continuously energized by the controlling current from the windshield wiper drive controller 4. As a result, the electromagnetic relay 2 connects the movable contact AR to the normally open contact N/O to permit the direct current to flow through the DC motor 1 continuously as shown by the solid-line arrow I in FIG. 1. Therefore, the windshield wiper can be driven continuously.
When the windshield wiper switch 5 connects the movable contact 5m to the fixed contact 5a (xe2x80x9cOFFxe2x80x9d position), the coil 2C of the electromagnetic relay 2 is not energized so that the electromagnetic relay 2 connects the movable contact AR to the normally closed contact N/C. Therefore, the DC motor 1 becomes a direct current generator to allow current to flow through the DC motor 1 in the direction opposite to the direction in which the direct current flows as shown by the solid-line arrow I in FIG. 1. Thus, the DC motor 1 can be braked and stopped.
Next, an example of a conventional DC motor drive circuit for use in a power window drive section will be described next with reference to FIG. 2.
Referring to FIG. 2, one end of a power window DC motor 11 is connected to a movable contact terminal 12a of an electromagnetic relay 12 that can move the power window upward. The other end of the DC motor 11 is connected to a movable contact terminal 13a of an electromagnetic relay 13 that can move the power window downward.
A normally closed contact terminal 12b of the electromagnetic relay 12 and a normally closed contact terminal 13b of the electromagnetic relay 13 are connected to each other. A connection point 12d between the normally closed contact terminal 12b and the normally closed contact terminal 13b is connected to the ground. A normally open contact terminal 12m of the electromagnetic relay 12 and a normally open contact terminal 13m of the electromagnetic relay 13 are connected to each other. A connection point 12e between the normally open contact terminal 12m and the normally open contact terminal 13m is connected to the power supply at the terminal 3, at which a positive DC voltage (+B) is connected from a car battery, for example.
The coil 12C of the electromagnetic relay 12 is energized by controlling current supplied from a power window ascending controller 14 when a user operates the power window drive section to move the power window upward. The coil 13C of the electromagnetic relay 13 is energized by controlling current supplied from a power window descending controller 16 when the user operates the power window drive section to move the power window downward.
Specifically, while the user is operating the power window drive section to move the power window upward, a switch 15 is continuously energized so that the coil 12C of the electromagnetic relay 12 is energized by the controlling current from the power window ascending controller 14, permitting the electromagnetic relay 12 to connect the movable contact AR to the normally open contact N/O. Therefore, a DC current flows through the DC motor 11 in the direction shown by a solid-line arrow I1 in FIG. 2 and thereby the DC motor 11 can be driven in the positive direction, for example. Therefore, the power window of the automobile can be moved upward, i.e. in the power window closing direction.
When the user stops operating the power window drive section to move the power window upward, the switch 15 is de-energized so that the coil 12C of the electromagnetic relay 12 is not energized by the control current, permitting the electromagnetic relay 12 to connect the movable contact AR to the normally closed contact N/C. As a result, the DC motor 11 can be braked and thereby the upward movement of the power window can be stopped.
While the user is operating the power window drive section to move the power window downward, a switch 17 is continuously energized so that the coil 13C of the electromagnetic relay 13 is energized by the controlling current from the power window descending controller 16 to permit the electromagnetic relay 13 to connect the movable contact AR to the normally open contact N/O. Therefore, direct current flows through the DC motor 11 in the direction shown by a dashed-line arrow I2 in FIG. 2 and the DC motor 11 can be driven in the opposite direction. Thus, the power window can be moved downward, i.e. in the power window opening direction.
When the user stops operating the power window drive section to move the power window downward, the switch 17 is de-energized so that the coil 13C of the electromagnetic relay 13 is not energized by the control current, permitting the electromagnetic relay 13 to connect the movable contact AR to the normally closed contact N/C. Therefore, the DC motor 11 can be braked and thereby the downward movement of the power window can be stopped.
In this manner, the conventional DC motor drive circuit uses one contact group of the electromagnetic relay and energizes the coil of the electromagnetic relay to connect the movable contact AR to the normally open contact N/O to drive the DC motor. On the other hand, the conventional DC motor drive circuit de-energizes the coil of the electromagnetic relay to connect the movable contact AR to the normally closed contact N/C to brake the DC motor.
In the electromagnetic relay used in this kind of DC motor drive circuit, while the coil is being de-energized to release the electromagnetic relay since direct current has flowed to the DC motor through the normally open contact N/O of the electromagnetic relay, when the movable contact AR separates from the normally open contact N/O, an arc occurs between the normally open contact N/O and the movable contact AR. If a gap length between the movable contact AR and the normally open contact N/O in the released state of the electromagnetic relay (this gap length will hereinafter be referred to as a xe2x80x9ccontact gap lengthxe2x80x9d for simplicity) is not sufficient, when the electromagnetic relay is released, the movable contact AR comes in contact with the normally closed contact N/C before the arc occurring between the normally open contact N/O and the movable contact AR is cut off. As a consequence, the normally closed contact N/C and the normally open contact N/O of the contact group are short-circuited (shorted). Unavoidably, the electromagnetic relay will be degraded and some suitable circuit elements such as a control circuit mounted on the same printed circuit board as this electromagnetic relay will be destroyed.
To overcome the above-mentioned disadvantages encountered with the prior-art electromagnetic relay, the contact gap length has hitherto been determined in accordance with the value of voltage (value of battery voltage) applied to the power supply at the terminal 3. Ordinary automobiles can be activated by a standard car battery of DC 12V and are able to drive the above DC motor drive circuit by an electromagnetic relay having a contact gap length of 0.3 mm, for example. Large automobiles such as a truck and a bus can be activated by a car battery of a high voltage higher than 24V (maximum voltage value is 32V), for example. Therefore, such large automobiles require an electromagnetic relay in which the contact gap length is longer than 1.2 mm, for example, to drive the above DC motor drive circuit.
Therefore, according to the prior art, since the contact gap length increases as the power supply voltage increases, it is unavoidable that the electromagnetic relay becomes large in size. Such large electromagnetic relay becomes troublesome when it is mounted on the printed circuit board. Moreover, since the stroke of the movable contact AR of such large electromagnetic relay lengthens, it is unavoidable that an operating speed of an electromagnetic relay decreases. In particular, recently, as so-called hybrid cars, which can be driven by an engine using electricity together with gasoline and electric cars become commercially available on the market, the voltage of the car battery becomes high increasingly. Therefore, the above-mentioned problem becomes considerably serious.
In view of the aforesaid aspects, it is an object of the present invention to provide an electromagnetic relay in which an arc cut-off capability can be improved without increasing a contact gap length.
In this specification, a capability of an electromagnetic relay for cutting off an arc occurred when a movable contact of an electromagnetic relay separates from a normally open contact before the movable contact is connected to the normally closed contact will be referred to as an xe2x80x9carc cut-off capabilityxe2x80x9d.
It is another object of the present invention to provide a DC motor drive circuit using this electromagnetic relay in which a short-circuit caused by an arc can be avoided even when a high power supply voltage is applied to the electromagnetic relay.
According to an aspect of the present invention, there is provided an electromagnetic relay which is comprised of a coil and a contact group containing a plurality of normally open contacts which are connected in series when the contact group is switched under electromagnetic control of the coil.
In accordance with another aspect of the present invention, there is provided an electromagnetic relay which is comprised of a coil, a normally closed contact, a plurality of movable contacts containing a movable contact which is connected to the normally closed contact when the coil is not energized, a plurality of normally open contacts provided in correspondence with a plurality of movable contacts and an armature driven under electromagnetic control effected when the coil is energized to thereby simultaneously displace a plurality of movable contacts so that a plurality of movable contacts are connected to a plurality of normally open contacts.
According to the DC motor drive circuit using the inventive electromagnetic relay thus arranged, when the coil of the electromagnetic relay is energized by the control current in order to drive the DC motor and the electromagnetic relay connects its movable contact to the normally open contact to permit the direct current to be supplied to the DC motor, the direct current is supplied through a plurality of normally open contacts connected in series to the DC motor.
Accordingly, since a circuit voltage obtained when the electromagnetic relay is released after the supply of control current to the coil of the electromagnetic relay has been stopped is applied to a plurality of gaps between the movable contacts (the movable contact is connected to the normally closed contact when the electromagnetic relay is fully released) and the normally open contacts connected in series, the voltage applied to each gap is divided by the number of the normally open contacts connected in series and therefore decreases.
Therefore, when the supply of control current to the coil of the electromagnetic relay is stopped and the electromagnetic relay is released, even if the arc occurs between the movable contact and the normally open contact N/O, the voltage applied to each of a plurality of gaps between the movable contacts and the normally open contacts connected in series decreases so that the problem of short caused by the arc can be solved even though the contact gap length is reduced.
According to the electromagnetic relay of the present invention, a plurality of movable contacts separate from a plurality of normally open contact N/O connected in series at the same time and therefore the separating speed of the movable contact can increase equivalently.
As described above, according to the present invention, since a plurality of normally open contacts, each having a short contact gap length, are connected in series so that the length of contact gap to which the power supply voltage is applied can increase equivalently, even when the electromagnetic relay with the short contact gap length is used, the arc occurring when the movable contact of the electromagnetic relay separates from the normally open contact can be cut off before the movable contact is returned to the normally closed contact side. Specifically, even the electromagnetic relay with the short contact gap length can improve the arc cut-off capability.
As set forth above, according to the electromagnetic relay of the present invention, since the arc cut-off capability of the electromagnetic relay is improved, even when a power supply voltage of a circuit increases, there can be used the electromagnetic relay whose contact gap length is reduced.
Furthermore, according to the electromagnetic relay of the present invention, since a plurality of normally open contacts are connected in series within a single electromagnetic relay, fluctuations of timing at which the movable contact separate from these normally open contacts connected in series can be decreased with ease and therefore the arc cut-off capability can be improved much more.